The much-publicised $765 million settlement of NFL concussion claims in the USA has been rejected by a federal judge who has said it may not be enough to cover 20,000 retired players who may be affected.

The latest development will be closely monitored by rugby authorities who have had to take a far more proactive approach to the subject on concussion as awareness has increased as a result of the US settlement.

A week after lawyers representing former NFL players filed a detailed payout plan, US District Judge Anita Brody said she needed more financial analysis.

"I am primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis or their [families] ... will be paid," she said.

Under proposed terms, players affected would receive between $25,000 and $5 million depending on their age and the severeness of their illness. The settlement includes $675 million for compensatory claims for players with neurological symptoms; $75 million for baseline testing for asymptomatic men; and $10 million for medical research and education. The NFL also would pay an additional $112 million to the players' lawyers for their fees and expenses, for a total payout of nearly $900 million.

More than 4500 former players or family members have filed suit, some accusing the NFL of fraud for its handling of concussions.

As rugby's approach to the issue has come under the spotlight, with claims baseline tests are regularly manipulated by clubs and players, the RFU announced on Monday there would be compulsory education for all players, coaches, referees and medics in England from the start of next season.